1. Field of the Invention
The present invention relates to a semiconductor angular velocity determining device mounted on automobiles and the like for determining a rotational angular velocity.
2. Description of the Related Art
FIG. 16 is an exploded perspective view of a conventional semiconductor angular velocity determining device disclosed in Japanese Patent Laid Open No. 5-322579, FIG. 17 is a exploded perspective cross sectional view taken along the line A--A of FIG. 16, FIG. 18 is a plan view of the interior of the semiconductor angular velocity determining device of FIG. 16, and FIG. 19 is a cross sectional view taken along the line B--B of FIG. 18.
The semiconductor angular velocity determining device includes a piezoelectric substance 1 as an oscillation driving source, a first substrate 2 bonded to the piezoelectric substance 1, composed of a glass as an insulating material and having a rectangular recess 2a, a detector main body 4 mounted on the first substrate 2 and composed of a silicon material having a (110) oriented plane, a second substrate 6 mounted on the detector main body 4, composed of glass as an insulating material and having a rectangular recess 5, and a first substrate detecting electrode 3 disposed in the recess 2a.
The detector main body 4 includes a generally T-shaped oscillator 9 composed of a beam portion 8 and a support portion 7 to which wirings (not shown) are connected and formed, the beam portion 8 being flexibly deformable and capable of oscillation and the support portion 7 being bonded on the first substrate 2 at both the ends thereof, a first detecting electrode 10 and a second detecting electrode 11 disposed on either side of and parallel to the beam portion 8, and a frame 12 disposed to surround the oscillator 9, the first detecting electrode 10 and the second detecting electrode 11. The first substrate detecting electrode 3 and the beam portion 8 constitute a detecting capacitor 70.
FIG. 20 is an electric circuit diagram of the semiconductor angular velocity determining device in which an inverter 100 is connected in parallel to an amplifier 101. A reference capacitor 102 having a given reference capacitance C.sub.1 is connected in series to the inverter 100 and a detecting capacitor 103 having a variable detecting capacitance C.sub.2 is connected in series to the amplifier 101. Further, the reference capacitor 102 and the detecting capacitor 103 are connected to a comparator 104. Cs in the drawing denotes a parasitic capacitance and when a clock signal is input to the electric circuit, an output voltage is supplied which is approximately proportional to (C.sub.1 -C.sub.2)/(C.sub.1 +C.sub.2 +2.times.Cs).
Next, operation of the above semiconductor angular velocity determining device will be briefly described. When the beam portion 8 is oscillated in the direction of the first detecting electrode 10 and the second detecting electrode 11 (along width direction of the beam portion 8) by the piezoelectric substance 1, the oscillation driving source, the beam portion 8 makes a simple harmonic motion centered on the support portion 7 so that the distances of a first gap 13 between the beam portion 8 and the first detecting electrode 10 and a second gap 14 between the beam portion 8 and the second detecting electrode 11 change. As a result, the capacitances of the first gap 13 and the second gap 14 change and the maximum velocity of oscillation of the beam portion 8 is controlled so that it remains constant at all times by the values of the capacitances, that is, the velocity is controlled so that it remains constant at all times when the beam portion 8 is located in parallel with the first detecting electrode 10 and the second detecting electrode 11.
When a rotational angular velocity is applied to the oscillator 9 with the longitudinal direction of the beam portion 8 as a rotational axis system at the time the beam portion 8 makes the simple harmonic motion along the plane of the first substrate 2 centered on the support portion 7, a Coriolis force, which acts on the first substrate 2 in a vertical direction, is applied to the beam portion 8. As a result, the extreme end of the beam portion 8 is displaced along an elliptical locus by the combined force resulting from the Coriolis force and a force acting on the beam portion 8 along the first substrate 2, thus a third distance 15 between the beam portion 8 and the first substrate detecting electrode 3, that is, the detecting capacitance C.sub.2 of the first detecting capacitor 70 composed of the beam portion 8 and the first substrate detecting electrode 3 changes.
In addition, a reference capacitor 102 is disposed externally which has a given reference capacitance C.sub.1 and whose dielectric substance is composed of ceramic. An output voltage which is approximately proportional to (C.sub.1 -C.sub.2)/(C.sub.1 +C.sub.2 +2.times.Cs) is obtained by comparing the reference capacitance C.sub.1 and the detecting capacitance C.sub.2 of the first detecting capacitor 70, and the output signal is input to a computer unit to determine the rotational angular velocity of the oscillator 9.
It is well known that a condition necessary for accurate determination of rotational angular velocity is that the resonance frequency of the oscillator 9 in the oscillating direction and the resonance frequency thereof in the direction of the Coriolis force be close to one another and it is preferable to form the oscillator 9 to a predetermined shape for this purpose.
In the conventional semiconductor angular velocity determine device, the oscillator 9, the first detecting electrode 10 and the second detecting electrode 11 are made by forming, for example, a rectangular mask shown by the dot-dash line in FIG. 21 on a single silicon sheet bonded to the first substrate 2 and etching the mask.
When the oscillator 9 is made by anisotropically etching a silicon sheet having a (110) oriented plane, the dimension of the oscillator 9 in an oscillating direction, that is, the dimension thereof in the width direction of the beam portion 8 and the dimension of the oscillator 9 in the Coriolis force direction, that is, the dimension thereof in the thickness direction of the beam portion 8 can be controlled with a great accuracy. However, because of the high etching rate the support portion 7 is another matter. Support portion 7 in an actually made oscillator 9 is not symmetrical with respect to the center axis line of the beam portion 8 as shown by the solid line in FIG. 21 and further the oscillation origin points 16 of the beam portion 8 are also not located symmetrically with respect to the center axis line of the beam portion 8.
Thus, because the shape and dimensions of the oscillator 9 obtained by etching cannot be managed with any high accuracy, it is difficult to cause the resonance frequency of the oscillator 9 in the oscillating direction to coincide with the resonance frequency thereof in the Coriolis force direction and so rotational angular velocity cannot be determined accurately.
The reference capacitance C.sub.1 is obtained by the reference capacitor 102 whose dielectric substance is composed of ceramic, whereas the detecting capacitance C.sub.2 of the detecting capacitor 70 is obtained by the detecting capacitor 70 whose dielectric substance is composed of air. Therefore, since the material of the dielectric substance of the reference capacitor 102 is different from that of the detecting capacitor 70, a highly accurate rotational angular velocity cannot be determined because the dielectric constants of the respective dielectric substances have different temperature characteristics.
Further, since the reference capacitor 102 is disposed externally, wirings are needed to electrically connect the reference capacitor 102 to the detecting capacitor 70. However, there is a problem that noise is liable to be multiplexed with a signal through the wirings.
An object of the present invention is to solve the above problems to provide a semiconductor angular velocity determining device capable of enhanced sensitivity and accuracy.